Manufacturing apparatus and manufacturing method of porous glass base material for optical fiber

ABSTRACT

A manufacturing apparatus of a porous glass base material for optical fiber includes: a liquid mass flow controller for controlling a flow rate of a raw material liquid of an organic siloxane; a vaporizer for mixing the raw material liquid and a carrier gas to vaporize the raw material liquid to form a mixed gas in which a raw material gas and the carrier gas are mixed; a raw material liquid nozzle for ejecting the raw material liquid into the vaporizer; a carrier gas supply pipe for supplying the carrier gas into the vaporizer; a raw material liquid pipe for introducing the raw material liquid into the raw material liquid nozzle; a burner for combusting the mixed gas together with a combustible gas and a combustion supporting gas to produce SiO 2  fine particles; a mixed gas pipe; an open/close valve; and a purge gas supply pipe.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) from Japanese Patent Application No. 2019-109084, filed on Jun.11, 2019, the entire contents of which are incorporated herein byreference.

BACKGROUND Technical Field

The present invention relates to a manufacturing method of porous glassbase material for an optical fiber using organic siloxane as a rawmaterial.

Background Art

A preforms for optical fiber is manufactured, for example, by externallydepositing SiO₂ fine particles by the OVD method or the like on a corebase material manufactured by the VAD method or the like and sinteringthe deposited body. As a silicon compound raw material for externallydepositing the SiO₂ fine particles on the core base material, an organicsilicon compound containing no Cl (chlorine) in molecules is sometimesused as a starting raw material of the SiO₂ fine particles. An exampleof such an organic silicon compound is octamethylcyclotetrasiloxane(OMCTS), which is a high-purity organic siloxane available on anindustrial scale. JP-2019-073406A discloses a technique for supplyingthe OMCTS to a burner and for synthesizing glass fine particles inoxyhydrogen flame.

When OMCTS is used as the starting raw material, SiO₂ fine particles areproduced by the reaction shown below.[SiO(CH₃)₂]₄+16O₂→4SiO₂+8CO₂+12H₂O

As described above, when the halogen-free organic siloxane typified byOMCTS is used as the silicon compound raw material supplied to theburner, hydrochloric acid is not discharged. For this reason, there isan advantage that the degree of freedom in the material of themanufacturing apparatus and the handling of exhaust gas is high.

The deposition of the SiO₂ fine particles using organic siloxane as astarting material is performed as follows, for example.

First, the organic siloxane raw material liquid is introduced into avaporizer while the flow rate is controlled by a liquid mass flowcontroller. Subsequently, the raw material liquid injected from a rawmaterial liquid nozzle and the carrier gas are mixed in the vaporizer,and the raw material liquid is vaporized to generate a raw material gasin a gaseous state. The raw material gas mixes with the oxygen gasdownstream of the vaporizer to form a raw material mixed gas. The rawmaterial mixed gas, combustible gas, and combustion supporting gas aresupplied to a burner, where SiO₂ fine particles are produced bycombustion reaction.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The produced SiO₂ fine particles are deposited on the core basematerial. After depositing a predetermined amount of SiO₂ fineparticles, the supply of the raw material to the vaporizer is stopped.At this time, the raw material liquid is accumulated in a raw materialliquid pipe and the raw material liquid nozzle downstream of the liquidmass flow controller, and a polymerization product and a gel-likesubstance are likely to be produced at the tip of the raw materialliquid nozzle in the vaporizer heated above the boiling point of the rawmaterial liquid.

The present invention has been made in view of the above-mentionedproblems, and an object of the present invention is to prevent thepolymerization product or the gel-like substance from being produced atthe tip of the raw material liquid nozzle in the vaporizer while supplyof the raw material is stopped with respect to organic siloxane rawmaterials typified by octamethylcyclotetrasiloxane (OMCTS).

Means for Solving the Problems

That is, in order to solve the above problems, a porous glass basematerial manufacturing apparatus for an optical fiber according to thepresent invention includes: a liquid mass flow controller forcontrolling a flow rate of a raw material liquid of an organic siloxane;a vaporizer for mixing the raw material liquid and a carrier gas tovaporize the raw material liquid to form a mixed gas in which a rawmaterial gas and the carrier gas are mixed; a raw material liquid nozzlefor ejecting the raw material liquid into the vaporizer; a carrier gassupply pipe for supplying the carrier gas into the vaporizer; a rawmaterial liquid pipe for introducing the raw material liquid suppliedfrom the liquid mass flow controller into the raw material liquidnozzle; a burner for combusting the mixed gas together with acombustible gas and a combustion supporting gas to produce SiO₂ fineparticles; a mixed gas pipe for supplying the mixed gas to the burner;an open/close valve provided on a flow path of the raw material liquidpipe; and a purge gas supply pipe that joins the raw material liquidpipe between the open/close valve and the raw material liquid nozzle tosupply a purge gas.

The manufacturing apparatus according to the present invention mayfurther include means for adjusting the flow rate of the purge gassupplied to the purge gas supply pipe. The manufacturing apparatus mayfurther comprise a means for adjusting the flow rate of the carrier gassupplied to the carrier gas supply pipe.

The manufacturing apparatus according to the present invention mayfurther include an oxygen gas supply pipe that joins an oxygen gas inthe middle of the mixed gas pipe. In this instance, when the flow rateof the purge gas supplied from the purge gas supply pipe is L_(P) andthe flow rate of the oxygen gas supplied from the oxygen gas supply pipeis L_(O2), the purge gas and the oxygen gas may be supplied from thepurge gas supply pipe and the oxygen gas supply pipe, respectively, sothat L_(O2)/L_(P)>14 is satisfied, in a state in which the open/closevalve is closed and the supply of the raw material is stopped.

In the present invention, when the flow rate of the purge gas suppliedfrom the purge gas supply piping is L_(P) and the flow rate of thecarrier gas supplied from the carrier gas supply pipe is L_(C), thepurge gas and the carrier gas may be supplied from the purge gas supplypipe and the carrier gas supply pipe, respectively, so thatL_(C)/L_(P)>10 is satisfied, in a state in which the open/close valve isclosed and the supply of the raw material is stopped.

Further, a manufacturing method of a porous glass base material foroptical fiber according to the present invention comprises steps of:controlling a flow rate of a raw material liquid of an organic siloxaneby a liquid mass flow controller; introducing the raw material liquidsupplied from the liquid mass flow controller to a raw material liquidnozzle of a vaporizer by a raw material liquid pipe; ejecting the rawmaterial liquid from the raw material liquid nozzle into the vaporizer;mixing the raw material liquid ejected in the vaporizer and the carriergas to vaporize the raw material liquid to form a mixed gas in which theraw material gas and the carrier gas are mixed; supplying the mixed gasto a burner; combusting the mixed gas together with a combustible gasand a combustion supporting gas in the burner to produce SiO₂ fineparticles; depositing SiO₂ fine particles on a starting core basematerial to form the porous glass base material for optical fiber;closing an open/close valve provided on a flow path of the raw materialliquid pipe to stop the supply of the raw material liquid from theliquid mass flow controller; and supplying a purge gas from a purge gassupply pipe that joins the raw material liquid pipe between theopen/close valve and the raw material liquid nozzle to the raw materialliquid pipe.

The manufacturing method according to the present invention may furtherinclude a step of additionally mixing oxygen gas into the mixed gas andsupplying the mixed gas to the burner. In this instance, when the flowrate of the purge gas supplied from the purge gas supply pipe is L_(P)and the flow rate of the oxygen gas additionally mixed is L_(O2), in thestep of supplying the purge gas to the raw material liquid pipe, thepurge gas and the oxygen gas may respectively be supplied so thatL_(O2)/L_(P)>14 is satisfied.

In the manufacturing method according to the present invention, when theflow rate of the purge gas supplied from the purge gas supply pipe isL_(P) and the flow rate of the carrier gas supplied to the vaporizer isL_(C), in the step of supplying the purge gas to the raw material liquidpipe, the purge gas and the carrier gas may respectively be supplied sothat L_(C)/L_(P)>10 is satisfied.

In the present invention, the carrier gas may be any of nitrogen, argon,and helium. The organic siloxane may be octamethylcyclotetrasiloxane(OMCTS).

Effect of the Invention

According to the present embodiment, it is possible to prevent thepolymerization product or the gel-like substance from being produced atthe tip of the raw material liquid nozzle with respect to organicsiloxane raw materials typified by octamethylcyclotetrasiloxane (OMCTS).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a supply flow diagram around the vaporizer in themanufacturing apparatus of the porous glass preform for optical fiberaccording to the present embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail, but the present invention is not limited thereto.

FIG. 1 is a supply flow diagram around a vaporizer in the manufacturingapparatus of the porous glass preform for optical fiber according to thepresent embodiment. A flow rate of a raw material liquid 101 containedin a raw material tank 100 is controlled by a liquid mass flowcontroller 1, and the raw material liquid 101 is supplied to a vaporizer2 through a raw material liquid pipes 101 a and 101 b. The raw materialliquid 101 is ejected from the tip of a raw material liquid nozzle 3 inthe vaporizer 2, and is formed into fine droplets by the carrier gas 102introduced into the vaporizer 2, and is heated. As a result, the rawmaterial liquid 101 is vaporized to form a mixed gas 103 in which theraw material gas and the carrier gas 102 are mixed. The mixed gas 103 issupplied to a burner 4 via a mixed gas pipe 103 a. The mixed gas 103 iscombusted together with a combustible gas 104 and a combustionsupporting gas 105 to produce SiO₂ fine particles. At this time, inorder to promote the combustion of the raw material gas, an oxygen gas106 supplied from an oxygen gas supply pipe 106 a may be joined in themiddle of the mixed gas pipe 103 a, and the mixed gas may be mixed withthe oxygen gas 106 before being supplied to the burner 4. The burner 4is reciprocated relatively along a starting core base material by areciprocating moving unit (not shown) while producing the SiO₂ fineparticles as described above, thereby depositing externally on thestarting core base material.

The temperature of the vaporizer 2 is set based on the viewpoint ofefficiently vaporizing the raw material liquid 101 and preventingpolymerization of the raw material liquid 101. When OMCTS is used as theorganic siloxane raw material, it is preferable to set the temperatureof the vaporizer 2 to a temperature of 160° C. or more and 220° C. orless. When the temperature is low, the vapor pressure of the rawmaterial liquid is lowered, and when the temperature is lower than 160°C., the vaporization efficiency significantly decreases. When thetemperature exceeds 220° C., the polymer derived from the raw materialliquid 101 may be deposited particularly near the raw material liquidnozzle 3. Further, the mixed gas pipe 103 a up to the burner 4 arrangeddownstream of the vaporizer 2 is preferably heated to a temperaturehigher than the liquefaction temperature of the raw material gas so thatthe raw material gas in the mixed gas 103 is not liquefied. Theliquefaction temperature of the raw material gas can be obtained byexperiments, and the liquefaction temperature T can also be easilyobtained by reverse calculation from Antoine equation (equation (1))using the raw material gas partial pressure Ps obtained from thepressure in the pipe and the molar ratio of the raw material componentsin the mixed gas 103.

$\begin{matrix}{{\log_{10}P_{S}} = {A - \frac{B}{T + C}}} & (1)\end{matrix}$

In the above equation, A, B, and C are coefficients obtained accordingto the raw material species.

For example, when organic siloxane OMCTS is used as a raw material,coefficients of A=8.8828, B=1358.7, and C=175.06 are known. If thesecoefficients and the OMCTS partial pressure of 30 kPa are used, T willbe 133° C. The equation (1) is merely an experimental equation to whichdata are fitted, and there is a possibility that the pipe temperaturedrops by several degrees instantaneously or locally due to a change inthe flow rate of the mixed gas 103 or the like. Therefore, in order toprevent re-liquefaction of the raw material gas with a margin, it isactually preferable to heat the mixed gas pipe 103 a to a temperature atleast 20° C. higher than the liquefaction temperature T obtained byusing the equation (1). Further, when the temperature of the mixed gaspipe 103 a exceeds 220° C., the polymer derived from the raw materialtends to deposit on the inner wall of the mixed gas pipe 103 a, andtherefore, it is preferable to suppress the temperature of the mixed gaspipe 103 a to 220° C. or less.

When the external deposition amount reaches a predetermined amount, theopen/close valve 5 provided between the raw material liquid pipe 101 aand the raw material liquid pipe 101 b is closed to stop the supply ofthe raw material liquid. Thereafter, the purge gas 107 is supplied fromthe purge gas supply pipe 107 a that joins the raw material liquid pipe101 b on the downstream side of the open/close valve 5, and the rawmaterial liquid 101 accumulated in the raw material liquid pipe 101 band the raw material liquid nozzle 3 is purged to the vaporizer 2. Asthe purge gas, an inert gas which hardly reacts with the raw materialliquid may be used, and for example, nitrogen, argon, helium, or thelike can be used.

Even after the raw material liquid 101 is purged, the carrier gas 102may continue to be supplied into the vaporizer 2. As a result, thepurged raw material liquid evaporates in the vaporizer 2 to become a rawmaterial gas, is entrained with the carrier gas 102, and is dischargedto the burner 4 through the mixed gas pipe 103 a. At this time, it ispreferable that the combustible gas 104 and the combustion supportinggas 105 are also supplied to the burner 4 to continue the combustionreaction until all the purged raw material liquid is evaporated. Bydoing so, it is possible to prevent the unreacted raw material gas fromcondensing and contaminating the inner wall of a deposition container.At this time, the SiO₂ fine particles ejected from the burner 4 due tothe purged raw material liquid may be further deposited on the outerperiphery of the porous glass base material.

After the supply of the raw material liquid 101 is once stopped and themanufacturing of the base material is completed, when the open/closevalve 5 is opened and the supply of the raw material liquid 101 isrestarted in order to produce the next base material, it takes timeuntil the raw material liquid 101 completely replaces the purge gasremaining in the raw material liquid pipe 101 b and the raw materialliquid nozzle 3. Therefore, it is preferable that the volume of theportion of the raw material liquid pipe 101 b and the raw materialliquid nozzle 3 to be purged by the purge gas be as small as possible.When OMCTS is used as the raw material, the volume of the raw materialliquid pipe 101 b and the raw material liquid nozzle 3 to be purged isV₁ [ml], and the full range flow rate of liquid mass flow controller forthe raw material is F [g/min], it is preferable that (0.95×60×V₁)/F<10is satisfied. It is more preferable that (0.95×60×V₁)/F<5 is satisfied.It is further more preferable that (0.95×60×V₁)/F<1 is satisfied. Notethat 0.95 [g/ml] is the specific gravity of OMCTS at normal temperature.

When generalized to a material other than OMCTS (specific gravitythereof is S), it is preferable that (S×60×V₁)/F<10 is satisfied. It ismore preferable that (S×60×V₁)/F<5 is satisfied. It is further morepreferable that (S×60×V₁)/F<1 is satisfied.

In order to reduce the volume of the pipe, it may be narrowed innerdiameter of the raw material liquid pipe, or it may be shortened thelength of the raw material liquid pipe. When reducing the inner diameterof the raw material liquid pipe, in order to prevent the supply of theraw material liquid during manufacturing from being affected by increaseof the pressure, it is preferable to adjust the inner diameter and thelength of the raw material liquid pipe so that the pressure loss appliedto the raw material liquid pipe 101 b is 10 kPa or less when thefull-range flow rate F of the liquid mass flow controller for the rawmaterial is flowed. The pressure loss is more preferably 5 kPa or less,and more preferably 1 kPa or less.

In this manner, since the volume to be gas purged after the supply ofthe raw material is stopped becomes small, only the raw material liquidpipe 101 b and the raw material liquid nozzle 3 need to be replaced withthe raw material liquid 101 when the next supply of the raw material isstarted. Then, after the flow rate is controlled by the liquid mass flowcontroller 1, the raw material is supplied to the burner 4 in a shorttime. Immediately after the supply of the raw material is started, ifthe flow rate to be supplied is temporarily increased by the liquid massflow controller (for example, by setting it to the full range), thereplacement time can be further shortened.

After the deposition is completed, when the raw material liquid 101remaining in the raw material liquid pipe 101 b or the raw materialliquid nozzle 3 is purged after the raw material supply is stopped, ifthe flow rate of the purge gas 107 is too large with respect to the flowrate of the carrier gas, liquefaction or incomplete combustion is likelyto occur at the outlet of the burner 4. If an unreacted raw materialdroplet adheres to the surface of the porous glass base material underproduction due to liquefaction or incomplete combustion, or if thedensity of the soot layer to be deposited locally decreases, cracks mayoccur in the base material from this point as a starting point.Liquefaction occurs when the purged raw material liquid is not vaporizedsufficiently in the vaporizer 2 and is discharged as droplets toward theburner 4, or when the raw material components in the mixed gas aresupplied to the burner 4 in a high-concentration state and are condensedin the low temperature portion near the burner 4. The incompletecombustion occurs because the flow rate of the oxygen gas 106 and thecombustion supporting gas 105 supplied to the burner 4 is insufficientfor the raw material component in the mixed gas.

The raw material liquid 101 remaining in the raw material liquid pipe101 b and the raw material liquid nozzle 3 is extruded by the purge gas107 and ejected into the vaporizer 2, where it is vaporized. Since thevolume of the raw material gas vaporized in the vaporizer 2 is about 100times that of the supplied raw material liquid, in consideration ofthis, it is preferable to adjust the flow rate of the purge gas so asnot to excessively increase the flow rate of the purge gas with respectto the flow rate of the carrier gas in consideration of this. When theflow rate of the purge gas is L_(P) [SLM], the raw material liquidremaining in the raw material liquid pipe 101 b and the raw materialliquid nozzles 3 is extruded by the purge gas at the flow rate Q₁[g/min] according to the equation (2).Q ₁=0.95×1000×L _(P)  (2)

When the carrier gas flow rate is L_(C)[SLM], the saturated vaporpressure of the raw material gas (for example, vaporized OMCTS) at thevaporizer temperature T_(V) [° C.] is Ps [atm], the total pressure inthe vaporizer is P [atm], and the molecular weight of the raw materialis M [g/mol], the raw material flow rate Q₂ [g/min] is expressed by thefollowing equation (3).

$\begin{matrix}{Q_{2} = \frac{L_{C} \times M \times P_{S}}{22.4 \times \left( {P - P_{S}} \right)}} & (3)\end{matrix}$

When the volume of the raw material liquid pipe 101 b and the rawmaterial liquid nozzle 3 to be purged is V₁ [ml], in order to avoidinsufficient vaporization of the raw material extruded by the purge gas,at least from the start of flowing the purge gas until the time of1000V₁/L_(P) elapses, the purge gas flow rate L_(P) and the carrier gasflow rate L_(C) may be adjusted so as to satisfy the relationship ofQ₁<3×Q₂. The saturated vapor pressure Ps [atm] of the raw material(e.g., OMCTS) at the temperature T_(V) [° C.] in the vaporizer 2 isobtained from the equation (1). As shown in FIG. 1, the flow rateadjusting means 6 may be provided in the raw material liquid pipe 101 band the purge gas supply pipe 107 a for purging the raw material liquidnozzle 3. As the flow rate adjusting means 6, a regulating valve such asa needle valve may be used, or a mass flow controller for gas may beused. A pressure loss portion such as an orifice may be provided toadjust the pressure there before and thereafter. When the carrier gasflow rate is L_(C) and the volume of the raw material liquid pipe 101 band the raw material liquid nozzle 3 to be purged is V₁, at least fromthe start of flowing the purge gas until the time of V₁/L_(P) elapses,L_(C) is preferably adjusted to satisfy L_(C)/L_(P)>10, and morepreferably L_(C)/L_(P)>20. In addition, oxygen gas may be further mixedwith the mixed gas discharged from the vaporizer 2 toward the burner 4.Then, when the flow rate of the mixed oxygen gases is L₂, and the volumeof the vaporizer 2 is V₂, at least from the start of flowing the purgegas until the time of V₂/(L_(C)+L_(P)) elapses, the flow rates arepreferably adjusted to satisfy L_(O2)/L_(P)>14, and more preferablyL_(O2)/L_(P)>23.

As shown in FIG. 1, the raw material liquid pipe 101 b and the purge gassupply pipe 107 a for purging the raw material liquid nozzle 3 may beprovided with an open/close valve 7 upstream of the flow rate adjustingmeans 6. The open/close valve 7 is closed when the raw material liquid101 is supplied to the vaporizer 2, and is opened when the supply of theraw material is stopped and the gas purge is performed. By providing theopen/close valve 7 upstream of the flow rate adjusting means 6, when thegas purge, immediately after the open/close valve 7 is opened, thepressure of the purge gas from the upstream starts to be applied to theflow rate adjusting means 6, and the flow rate of the purge gasgradually increases. Therefore, it is easy to adjust L_(C)/L_(P) andL_(O2)/L_(P) to the predetermined ranges. Further, it is preferable toprovide a check valve 8 downstream of the flow rate adjusting means 6.By providing the check valve 8, it is possible to prevent the rawmaterial liquid 101 from flowing back to the purge gas supply pipe 107 awhen the raw material is supplied. An open/close valve may be used inplace of the check valve 8, in which case it is preferable to open theopen/close valve when the raw material supply is stopped, and close theopen/close valve when the raw material is supplied.

When the supply of the raw material is stopped, since the open/closevalve 5 is in a closed state, it is preferable to provide a raw materialliquid branch pipe 101 c that branches in the middle of the raw materialliquid pipe 101 a in order to prevent the raw material liquid 101 thathas accumulated in the raw material liquid pipe 101 a from the liquidmass flow controller 1 to the open/close valve 5 from causing a pressurefluctuation due to a change in ambient temperature. An open/close valve9 is provided in the raw material liquid branch pipe 101 c, and when theopen/close valve 5 is closed and the raw material supply is stopped, theopen/close valve 9 is opened. The end of the raw material liquid branchpipe 101 c may be connected to the raw material tank 100 as shown inFIG. 1, or may be connected to a recovery tank (not shown). Further,when the pressure in these tanks increases, it may be possible todepressurize using a back-pressure valve or the like. When theopen/close valve 5 is opened to supply the raw material, the open/closevalve 9 is closed so that the raw material liquid 101 is supplied onlyto the vaporizer 2 side.

EXAMPLES Example 1

OMCTS was used as the raw material liquid 101 of the organic siloxane,nitrogen (N₂) gas was used as the carrier gas 102, hydrogen (H₂) gas wasused as the combustible gas 104, and oxygen (O₂) gas was used as thecombustion supporting gas 105. The combustion supporting gas 105 wassupplied as a first combustion supporting gas and a second combustionsupporting gas, both of which were O₂ gases, the flow rates of whichwere independently settable. The first combustion supporting gas andsecond combustion supporting gas were ejected from different ejectionports in the burner. After the external deposition was completed, theflow rate of the raw material was reduced to 0 g/min. On the other hand,the gases other than the raw material continued to flow, and thedeposition surface portion of the porous glass base material was bakedand tightened by using a plurality of burners.

At this time, the flow rate L_(C) of the carrier gas 102 was set to 21SLM, the flow rate L_(O2) of the oxygen gas 106 to be additionally mixedwith the mixed gas 103 of the raw material gas and the carrier gas 102was set to 30 SLM, the flow rate of the combustible gas 104 was set to250 SLM, the flow rate of the first combustion supporting gas was set to25 SLM, and the second combustion supporting gas was set to 75 SLM. Atthe same time when the flow rate of the raw material became 0 g/min, theopen/close valve 5 was closed and the open/close valve 9 was opened.Thereafter, by opening the open/close valve 7, the raw material liquid101 remaining in the raw material liquid pipe 101 b and the raw materialliquid nozzle 3 was purged by the purge gas from the purge gas supplypipe 107 a. Nitrogen (N₂) gas was used as the purge gas 107, and thepurge gas was supplied at a flow rate L_(P) of 0.3 SLM. The temperatureof the vaporizer 2 was 200° C. When purging was performed under theabove conditions, incomplete combustion and liquefaction at the outletof the burner 4 could be prevented. In addition, it was possible toprevent polymerization products and gel-like substances from beingproduced at the tip portion of the raw material liquid nozzle 3.

Example 2

Except that the flow rate L_(P) of the purge gas 107 was 0.9 SLM, theexternal deposition and the baking and tightening of the depositionsurface portion were performed under the same conditions as inExample 1. As a result, incomplete combustion and liquefaction at theoutlet of the burner 4 at the time of purging could be prevented. Inaddition, it was possible to prevent polymerization products andgel-like substances from being produced at the tip portion of the rawmaterial liquid nozzle 3.

Example 3

Except that the flow rate L_(P) of the purge gas 107 was 1.5 SLM, theexternal deposition and the baking and tightening of the depositionsurface portion were performed under the same conditions as inExample 1. As a result, incomplete combustion occurred at the outlet ofthe burner 4 during purging. It was possible to prevent polymerizationproducts and gel-like substances from being produced at the tip portionof the raw material liquid nozzle 3.

Example 4

Except that the flow rate L_(P) of the purge gas 107 was 3.0 SLM, theexternal deposition and the baking and tightening of the depositionsurface portion were performed under the same conditions as inExample 1. As a result, incomplete combustion and liquefaction occurredat the outlet of the burner 4 during purging. It was possible to preventpolymerization products and gel-like substances from being produced atthe tip portion of the raw material liquid nozzle 3.

Table 1 shows the flow rate L_(P) of the purge gas 107, the flow rateL_(O2) of the oxygen gas 106 to be additionally mixed with the mixed gas103, the flow rate L_(C) of the carrier gas 102, andoccurrence/non-occurrence of incomplete combustion and liquefaction ineach example and comparative example.

TABLE 1 Incomplete Lp [SLM] L_(O2) [SLM] Lc [SLM] L_(O2)/Lp Lc/LpCombustion Liquefaction Example 1 0.3 21 30 70 100 no occurrence nooccurrence Example 2 0.9 21 30 23 33 no occurrence no occurrence Example3 1.5 21 30 14 20 occurrence no occurrence Example 4 3.0 21 30 7 10occurrence occurrence

As described above, in any of the examples, by purging the raw materialliquid 101 with the purge gas 107 while stopping the supply of the rawmaterial, it was possible to prevent the polymerization product or thegel-like substance from being produced at the tip portion of the rawmaterial liquid nozzle 3 in the vaporizer 2. From the viewpoint ofpreventing liquefaction at the outlet of the burner 4, it has been foundthat the flow rates of gases are preferably adjusted to satisfyL_(C)/L_(P)>10, and more preferable to satisfy L_(C)/L_(P)>20. From theviewpoint of preventing incomplete combustion, it has been found thatthe flow rates of gases are preferably adjusted to satisfyL_(O2)/L_(P)>14, and more preferable to satisfy L_(O2)/L_(F)>23.

What is claimed is:
 1. A manufacturing method of a porous glass basematerial for optical fiber comprising: controlling a flow rate of a rawmaterial liquid of an organic siloxane by a liquid mass flow controller;introducing the raw material liquid supplied from the liquid mass flowcontroller to a raw material liquid nozzle of a vaporizer by a rawmaterial liquid pipe; ejecting the raw material liquid from the rawmaterial liquid nozzle into the vaporizer; mixing the raw materialliquid ejected in the vaporizer and a carrier gas to vaporize the rawmaterial liquid to form a mixed gas in which the raw material gas andthe carrier gas are mixed; supplying the mixed gas to a burner;combusting the mixed gas together with a combustible gas and acombustion supporting gas in the burner to produce SiO₂ particles;depositing SiO₂ particles on a starting core base material to form theporous glass base material for optical fiber; closing an open/closevalve provided on a flow path of the raw material liquid pipe to stopthe supply of the raw material liquid from the liquid mass flowcontroller; and supplying a purge gas from a purge gas supply pipe thatjoins the raw material liquid pipe between the open/close valve and theraw material liquid nozzle to the raw material liquid pipe.
 2. Themanufacturing method according to claim 1 further comprises additionallymixing oxygen gas into the mixed gas and supplying the mixed gas to theburner.
 3. The manufacturing method according to claim 2, wherein when aflow rate of the purge gas supplied from the purge gas supply pipe isL_(P) and a flow rate of the oxygen gas additionally mixed is L_(O2) inthe supplying the purge gas to the raw material liquid pipe, the purgegas and the oxygen gas are respectively supplied so that L_(O2)/L_(P)>14is satisfied.
 4. The manufacturing method according to claim 1, whereinwhen a flow rate of the purge gas supplied from the purge gas supplypipe is L_(P) and a flow rate of the carrier gas supplied to thevaporizer is L_(C), in the supplying the purge gas to the raw materialliquid pipe, the purge gas and the carrier gas are respectively suppliedso that L_(C)/L_(P)>10 is satisfied.
 5. The manufacturing methodaccording to claim 1, wherein the carrier gas is any of nitrogen, argon,and helium.
 6. The manufacturing method according to claim 1, whereinthe organic siloxane is octamethylcyclotetrasiloxane (OMCTS).